A thermally expandable MS is obtained by microcapsulating a volatile foaming agent with a polymer and is also called a thermally expandable microcapsule or a thermally expandable microsphere. In general, the thermally expandable MS can be produced by a method in which a polymerizable monomer mixture containing at least one type of polymerizable monomers and a volatile foaming agent is suspension-polymerized in an aqueous dispersion medium. An outer shell (shell) is formed by a polymer formed as a polymerization reaction progresses, thereby obtaining the thermally expandable MS having a structure in which the foaming agent is encapsulated in the outer shell.
As the polymer forming the outer shell, a thermoplastic resin having good gas barrier properties is ordinarily used. The polymer forming the outer shell is softened by heating, and foamed. As the foaming agent, a low-boiling compound such as a hydrocarbon which becomes a gaseous state at a temperature equal to or lower than the softening point of the polymer forming the outer shell is typically used. When the thermally expandable MS is heated, the foaming agent vaporizes, and the expanding force thereof acts on the outer shell; however, at the same time, the elastic modulus of the polymer forming the outer shell rapidly decreases. Rapid expansion therefore occurs at a certain temperature serving as a boundary. This temperature is referred to as a foaming initiation temperature. When the thermally expandable MS is heated to a temperature equal to or higher than the foaming initiation temperature, the thermally expandable MS itself expands to form a foamed particle (closed-cell foamed particle). The expansion ratio refers to a value calculated by dividing a volume of foamed particles by a volume of an unfoamed thermally expandable MS.
However, thermally expandable MSs involve a problem of sag due to gas escaping after the foaming. The sag is a phenomenon in which, when a thermally expandable MS is heated, the outer shell first starts to soften and, at the same time, the foaming agent encapsulated therein starts to gasify to raise the internal pressure into an expanded state and, when the heating is further continued, the thermally expandable MS starts to shrink because gas passes and diffuses through the outer shell thinned by the expansion. The sag also relates to heat resistance of the thermally expandable MS and problems in elasticity, thermal stability, and the like.
Especially when the thermally expandable MSs are used to reduce weight, since the average particle size of the thermally expandable MSs is greater and a higher expansion ratio contributes better to the weight reduction, the thermally expandable MSs are required to have a large average particle size and a high expansion ratio. At the same time, from the perspective of ease in processing and molding, higher foaming initiation temperature and reduction in occurrence of sag have been demanded.
To produce the thermally expandable MS having a large particle size, Patent Document 1 describes a method using a colloidal silica aggregate. However, in Patent Document 1, a carboxyl group-containing monomer as a polymerizable monomer and a thermosetting resin are contained, and specifically, a methacrylic acid and a bisphenol A-type epoxy resin are contained without exception.
Patent Document 2 discloses a thermally expandable MS having excellent heat resistance and solvent resistance and excellent foamability in a temperature range of 200° C. or higher. However, as monomers, a nitrile-based monomer, a monomer having a carboxyl group, a monomer having an amide group, and a monomer having a cyclic structure in a side chain thereof are used without exception. Specifically, as the monomer having a carboxyl group, methacrylic acid is used. In the case of Patent Document 2, the average particle size is specifically from 12 to 30 μm.
Patent Document 3 discloses a thermally expandable MS that has high heat resistance and that reduces expansion initiation temperature when a heat treatment is performed at a temperature lower than the expansion initiation temperature. However, in Patent Document 3, the outer shell contains a copolymer obtained by polymerizing a polymerizable component containing a carboxyl group-containing monomer without exception. Specifically, methacrylic acid is used. In the case of Patent Document 3, the average particle size is specifically from 20 to 39 μm.
Patent Document 4 describes use of a bifunctional crosslinkable monomer to make a thermally expandable MS having the maximum expansion ratio of at least 5. However, the average particle size is approximately 30 μm and the maximum expansion ratio is specifically approximately from 30 to 60.
Patent Document 5 discloses a thermally expandable MS including isododecane as a foaming agent. However, the expansion ratio after 2 minutes of heat treatment at 170° C. and then 2 minutes of heating and forming at 200° C. was at most 46 in the examples, and the expansion ratio after 2 minutes of heat treatment at 170° C. and then 4 minutes of heating and forming at 200° C. was a little less than the expansion ratio of the case where 2 minutes of heating and foaming at 200° C. was employed.